rootfs-builder: add DEBIAN_FRONTEND=noninteractive

[Pennyzct]

Description of problem

Now, ARM CI failed on the following errors:
kata-containers/ci#276

05:09:13 Setting up tzdata (2019c-3ubuntu1) ...
05:09:13 debconf: unable to initialize frontend: Dialog
05:09:13 debconf: (TERM is not set, so the dialog frontend is not usable.)
05:09:13 debconf: falling back to frontend: Readline
05:09:13 Configuring tzdata
05:09:13 ------------------
05:09:13 
05:09:13 Please select the geographic area in which you live. Subsequent configuration
05:09:13 questions will narrow this down by presenting a list of cities, representing
05:09:13 the time zones in which they are located.
05:09:13 
05:09:13   1. Africa      4. Australia  7. Atlantic  10. Pacific  13. Etc
05:09:13   2. America     5. Arctic     8. Europe    11. SystemV
05:09:13   3. Antarctica  6. Asia       9. Indian    12. US
05:09:13 Geographic area: Build timed out (after 15 minutes). Marking the build as aborted.
05:24:13 Build was aborted

The interface of debconf is configured to Readline, and this frontend will prompt a series of questions, printed out at the console using plain text for users to choose.

Expected result

In container, we need to configure DEBIAN_FRONTEND=noninteractive, which will let user have zero interaction while installing or upgrading the system via apt. It accepts the default answer for all questions.